X-ray inspection methods use x-rays to penetrate an object to reveal its contents. In the past, these methods have relied upon differential absorption ratios (i.e., the ratio of absorption by a material of x-rays of two different energies) to accentuate different materials inside an object. This technique is useful with a video or “real time” inspection. Two images can be taken and one image “subtracted” from the other, thereby accentuating the difference. This technique usually requires an operator to take a video exposure, store it, change the x-ray source energy, take another exposure, and subtract. This can take several seconds. If the subject is moving, the operator may miss some of the inspection. Alternatively, the object to be inspected can be scanned twice at different energies. This requires the object to be rescanned and the two scans to be aligned perfectly.
The drawbacks inherent in these prior art x-ray inspection methods or techniques are overcome by the present invention, which provides a method of changing the energy of x-rays produced by a pulsed x-ray source so that successive x-ray pulses alternate between at least two different energy levels. The pulsed x-ray source basically comprises: an electron accelerator structure defining an electron flow path having an electron injection end; an electron gun having an electron source, a control grid, and optionally a feedback resistor connected to the electron source, wherein the electron gun is located at the injection end of the electron accelerator structure; and a microwave system connected to the electron accelerator structure, which includes a microwave power source and a pulse generator with a pulse-forming network, while the inventive method comprises:                selecting at least two different voltage levels; and between pulses generated by the pulsed x-ray source, alternating the voltage applied or delivered to the pulse-forming network of the pulse generator between the selected voltage levels; and                    (i) alternating the voltage applied or delivered to the control grid of the electron gun between the selected voltage levels, and/or            (ii) selecting at least two different resistance levels; and between pulses generated by the pulsed x-ray source, alternating the resistance through a feedback resistor on the electron gun between the selected resistance levels.                        
The present invention also provides a multiple energy x-ray source capable of rapidly generating and delivering x-rays in the form of successive pulses that alternate between at least two different energy levels, which comprises:                (a) an electron accelerator structure defining an electron flow path having an electron injection end;        (b) an electron gun having an electron source and a control grid and optionally a feedback resistor connected to the electron source, wherein the electron gun is located at the injection end of the electron accelerator structure for producing and delivering a stream of electrons to the electron injection end of the accelerator structure during pulses of predetermined length and of predetermined repetition rates;        (c) a microwave system connected to the electron accelerator structure that comprises: (i) a microwave power source; and (ii) a pulse generator with a pulse-forming network that is connected to the microwave power source;        (d) means for alternating the voltage applied or delivered to the pulse-forming network of the pulse generator between at least two different voltage levels; and        (e) means for alternating the voltage applied or delivered to the control grid of the electron gun, between at least two different voltage levels, and/or means for alternating the resistance through a feedback resistor on the electron gun between at least two different resistance levels.        
Further to the above, a method for inspecting moving objects is provided, which comprises:                (a) generating successive pulses of x-rays which alternate between at least a first and a second energy level, thereby forming a pulsed, multiple energy, x-ray beam;        (b) directing the pulsed, multiple energy, x-ray beam toward a moving object to be inspected;        (c) intercepting the pulsed, multiple energy, x-ray beam leaving the object and generating at least a first and a second signal or image therefrom; and        (d) processing the at least first and second signals or images generated for each inspected object, thereby allowing or permitting detection of different materials present therein.        
Also provided by way of the present invention is a multiple energy x-ray inspection apparatus for inspecting moving objects, which comprises:                (a) transport means for transporting objects for inspection through the apparatus;        (b) a multiple energy x-ray source for: generating successive pulses of x-rays which alternate between at least a first and a second energy level, thereby forming a pulsed, multiple energy, x-ray beam; and for directing same toward each object for inspection;        (c) sensor means for intercepting the pulsed, multiple energy, x-ray beam leaving each object and generating at least a first and a second signal or image therefrom; and        (d) processing means for processing the at least first and second signals or images generated for each inspected object (e.g., subtracting the signals or images to accentuate absorption differences), thereby allowing or permitting detection of different materials present therein.        
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and drawings. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.